Solar cells are devices that convert the energy of light into electricity using the photovoltaic effect. Solar power is an attractive green energy source because it is sustainable and produces only non-polluting by-products. Accordingly, a great deal of research is currently being devoted to developing solar cells with enhanced efficiency while continuously lowering material and manufacturing costs.
When light hits a solar cell, a fraction of the incident light is reflected by the surface and the remainder transmitted into the solar cell. The transmitted light/photons are absorbed by the solar cell, which is usually made of a semiconducting material, such as silicon. The absorbed photon energy excites electrons from the atoms of the semiconducting material, generating electron-hole pairs. These electron-hole pairs are then separated by p-n junctions and collected by conductive electrodes that are applied on the solar cell surface. These conductive electrodes typically comprise an electroconductive paste composition.
Traditional electroconductive pastes contain metallic particles, glass frit, and an organic vehicle. These components are usually selected to take full advantage of the theoretical potential of the resulting solar cell. The metallic particles act as the conductive component of the surface electrode. The organic vehicle provides the medium through which all of the components are combined. The glass frit component has a number of purposes, one of which is to improve contact between the electrode and the underlying silicon surface. It is desirable to maximize the contact between the electroconductive paste and silicon surface, as well as with the metallic particles themselves, so that the charge carriers can flow through the interface and then through the surface electrodes. The glass particles in the electroconductive paste provide the media by which the paste builds contact between the metal and the underlying silicon substrate. The glass component must have specific properties in order to achieve optimal contact. Thus, the goal is to minimize contact resistance while improving solar cell efficiency. Known glass compositions have high contact resistance due to the insulating effect of the glass in the interface of the electrode and silicon wafer. Further, glass frit is known to have wide melting temperature ranges, making their behavior strongly dependent on the processing parameters. Accordingly, electroconductive paste compositions with improved electrical properties are desirable. Specifically, electroconductive paste compositions with improved glass frit components are needed.
U.S. Patent Application Publication No. 2011/0308595 A1 discloses a thick-film paste for printing on the front-side of a solar cell device having one or more insulating layers. The thick-film paste comprises an electrically conductive metal, and lead-tellurium-oxide dispersed in an organic medium. The lead-tellurium-oxide is present in an amount of 0.5 to 15 wt. % of solids of the paste and the molar ratio of lead to tellurium is between 5/95 and 95/5. The lead-tellurium-oxide (Pb—Te—O) is prepared by mixing TeO2 and lead oxide powders, heating the powder mixture in air or an oxygen-containing atmosphere to form a melt, quenching the melt, grinding and ball-milling the quenched material, and screening the milled material to provide a powder with the desired particle size.
U.S. Pat. No. 5,066,621 (“'621 patent”) discloses a sealing glass composition comprising, in wt. %, 13-50% lead oxide, 20-50% vanadium oxide, 2-40% tellurium oxide, up to 40% selenium oxide, up to 10% phosphorous oxide, up to 5% niobium oxide, up to 20% bismuth oxide, up to 5% copper oxide and up to 10% boron oxide and an electrically conductive formulation comprising, in wt. %, 50-77% silver, 8-34% of a sealing glass composition as described previously, 0.2-1.5% resin and thixotrope and 10-20% organic solvent. The '621 patent discloses tellurium oxide in a preferred range of 9-30 wt. %.
U.S. Patent Application Publication No. 2011/0192457 (“'457 publication”) discloses an electroconductive paste containing an electro-conductive particle, an organic binder, a solvent, a glass frit, and an organic compound including alkaline earth metal, a metal with a low melting point or a compound affiliated with a metal with a low melting point. The '457 publication teaches the use of bismuth (Bi) containing glass frit and barium (Ba) containing glass frit.
U.S. Patent Application Publication No. 2010/0037951 discloses methods of making multi-element, finely divided, metal powders containing one or more reactive metals and one or more non-reactive metals. Reactive metals include metals or mixtures thereof such as titanium (Ti), zirconium (Zr), hafnium (Hf), tantalum (Ta), niobium (Nb), vanadium (V), nickel (Ni), cobalt (Co), molybdenum (Mo), manganese (Mn), and iron (Fe). Non-reactive metals include metals or mixtures such as silver (Ag), tin (Sn), bismuth (Bi), lead (Pb), antimony (Sb), zinc (Zn), germanium (Ge), phosphorus (P), gold (Au), cadmium (Cd), berrylium (Be), tellurium (Te).